It is typical for the customers (or subscribers) of a wireless service provider (carrier), hereinafter also referred to as the carrier's native subscribers, to roam outside of the territory served by that carrier (the home network), and onto foreign networks serviced by other wireless service providers. By definition, a foreign network may be in another country, requiring international roaming, or can simply be a network serviced by another carrier in the same country.
The home carrier negotiates agreements with various other carriers by which the home carrier compensates the foreign carrier via a tariff for accommodating its customers on the foreign networks. It is common for the terms of these roaming agreements to vary somewhat, such that inevitably, certain agreements are more favorable to the home carrier than others. According to some roaming protocols, subscribers that roam can register on the network that has the strongest signal. This may not be the carrier with the best roaming rates or the carrier that best supports enhanced services to support data products. Thus, the home carrier has an incentive to encourage use of certain of the foreign networks that are available to its roaming subscribers.
One approach to directing foreign network selection involves downloading a list of preferred networks in a particular region to the subscriber's mobile station, use of the list typically being facilitated by the subscriber identity module (SIM). However, if a preferred network is not available when the subscriber attempts to register in the region, the typical protocol may lead to selection and continued use of a non-preferred network until the subscriber leaves that region. To address this problem, another approach redirects, via instructions communicated over-the-air (OTA) or via short message service (SMS), for example, a roaming mobile station that has registered with a non-preferred or less optimal network by instructing the mobile station to switch to a preferred or optimal network.
An alternative approach serially rejects a registration attempt directed to non-preferred or less optimal networks until a preferred or more optimal network is found, until the list of available networks has been exhausted without finding a preferred network, or the registration process is rejected altogether. However, both of these approaches must first detect whether a native subscriber is roaming on a foreign network, which is accomplished by utilizing a high impedance probe to monitor a link between the networks to detect various location update and other signaling messages exchanged between the foreign network and the home network's subscriber database. As an external device introduced into the system to monitor all signaling messages in order to detect roamers, the reliability of such solutions is dependent upon the likelihood of probe failure and capacity limitations.
Furthermore, to avoid creating a negative user experience, the home carrier is wise to enforce its preferences without inconveniencing the subscriber that is roaming outside of the home network. The user experience is severely compromised by serial rejection of registration attempts, which will cause a significant delay in call setup time, and which may be followed by refusal of service if the attempt is finally rejected according to the redirection protocol.